The present invention relates to electronic flash control devices for measuring the amount of light reflected from a photographic object that is illuminated by flash light, and which interrupts flash firing while the flash tube is in operation when the amount of measured light reaches a given level by actuating a switch element serially connected with the flash tube.
Flash control devices of the above type are known and have been marketed by many manufacturers. For example, U.S. Pat. Re. 28,025 shows such a flash control device in FIGS. 3, 9 and 17 and an explanation of the structure and operation is set forth in the specification, and in particular the description of FIG. 17, beginning at Column 11, line 52, and continuing to Column 12, line 37. In accordance with the teachings of the aforesaid Reissue patent, a brightness output voltage A is compared with a set output voltage B, representing the film sensitivity and lens aperture settings, and when the two voltages are substantially equal, a stop signal pulse is generated, which is provided to the switching circuits illustrated in FIGS. 3, 9 and 17 for the purpose of discontinuing the operation of the flash device.
The switch circuits of FIGS. 3 and 9 are identical and utilize a first silicon controlled rectifier (SCR) connected in series with the flash device and which is controlled by a trigger circuit to enable actuation of the flash device. The stop or blocking signal from a comparator circuit, which compares the aforementioned signals A and B, is applied to the gate of a second SCR which causes current to bypass through a capacitor interconnected between the first and second SCRs so as to turn the flash off by turning the first SCR off.
The flash control circuit in the embodiment of FIG. 17 uses a delay circuit in lieu of the comparator circuits for turning off the flash device. A trigger delay circuit is actuated simultaneously with the actuation of the flash device. The delayed pulse output of the trigger circuit actuates the second SCR to discharge a capacitor connected such that with discharge of that capacitor a reverse current flows through the first SCR. The conduction of the second SCR causes a second capacitor to discharge in a manner to apply a reverse bias to the first SCR for the purpose of shortening the turnoff time thereof.
A flash device provided with such a control device, i.e. either a serially-switch-connected type or a direct stop type, has an advantage that the discharge of the main capacitor is stopped upon actuation of the switch element and the remaining charge in the main condenser is conserved for the next firing of the flash tube. However, if the photographic condition requires a large amount of flash light, for instance when the object to be photographed is distant from the camera, then the remaining capacitor charge is so small that much time is required for charging the main capacitor for the next flash operation. Hence, if photographs are repeatedly taken, for example by means of a motor driven still camera, with short periods or cycles with the above automatically controlled flash device under conditions that require a large amount of light, some pictures may be taken with insufficient flash light or without flash light that results, because the main capacitor, which has discharged a great deal, is incapable of being charged up to a sufficient level to provide an adequate amount of flash light or to fire the flash tube, in the short intervals between flashes.